Apparatus for measuring the thickness of metallic plating on bases



May 18, ,1943

S. ANDERSON Er AL APPARATUS FOR MEASURING THE THIcKNEss 0F METALLICPLATI NG ON BASES Filed July 24, 1939 2 Shee 11s-Sheet l Hlllll May 18,1943 s. ANDERSON ErAL 2,319,196

APPARATUS FOR MESURING THE THICKNESS OF METALLIC PLATING ON BASES FiledJuly 24, 1939 2 Sheets-Sheet 2 i ArmErE/e VoLrMErER n l .BATTERY 9 7Patented May 18, 1943 APPARATUS FOR MEASURING THE THICK- NESS OFMETALLIC PLATING ON BASES Stanley Anderson and Robert W. Manuel,Chicago, Ill., assignors to Crane Co., Chicago, Ill., a corporation ofIllinois i Application July 24, 1939, serial No. 286,158

1 Claim.

This invention relates to an apparatus and the method of using theapparatus for determining the thickness of metal plating and the like,as for example chromium and nickel plating. More particularly, theinvention relates to an apparatus and method for electrolyticallymeasuring the thickness of the plate at any location on the base whetherthe surface is plane or curved.

The' need for a convenient, rapid and accurate method of determining thelocal thickness, rather than the usual average thickness of metallicplatlng on a base, has long been recognized. At present the mostcommonly used method of checking plate thickness is that of removing theplate by dissolution in a chemical and subsequently analyzing thechemical solution. This method of measurement, if the area of thesurface from which the plate has been removed is precisely known, isaccurate only as a means of determining an average thickness of theplate. Obviously, on certain types of surfaces the distribution of theplate may be very uneven, and in such4 an event, it is much moredesirable to measure the thickness of the plate at locations where, byreason of the contour of the surface, it may have a tendency to depositlightly, or where, depending upon the article to which the plate hasbeen applied, it may be subjected to greater Wear than other portions ofthe plated surface. The invention described herein makes possible aconvenient, rapid and reasonably accurate determination of localthickness of such metallic plating.

One of the most important objects of the invention lies in the provisionof an instrument and method for electrolytically stripping a relativelysmall restricted and predetermined area of metallically plated surface,the said stripping taking place at a fixed or known rate whereby thetime required to strip the said area is directly proportional to thethickness of the plate upon the said surface. It is realized, of course,that stripping a plated surface electrolytically and measuring the timethus required is not basically new, but previously known methods havebeen applicable only to special types of surfaces such as, for example,flat or cylindrical, and have therefore lacked the convenience ofapplication of this invention.

Another object is to provide an apparatus which is capable ofreproducing a stripping area on successive measurements or which, if thearea is not susceptible to reproduction, is capable of reproducing thecurrent density by means of voltage regulation.

Another important object is to provide a method and means fordetermining plate thickness which does not necessitate the destructionof the article towhich the plating has been applied.

A still further object of the invention lies in the provision of aportable plating thickness measuring apparatus which is adaptable foruse upon substantially all sizes and shapes of plated articles.

Another object is to provide an apparatus which is capable ofsuccessively measuring the local thickness of a plurality of diierentmetal coatings deposited one upon another upon a base.

It is also an object of the invention to provide an electrolyticmetallic platestripping cell which produces a relatively uniform currentdistribution over the surface to be stripped of its plate whereby thestripping will proceed at an even rate. The same instrument can be usedfor determining thicknesses of various kinds of plating, it bef ingnecessary only to select a suitable electrolyte and cathode material.

Various other objects and advantages of the present invention will beapparent from the embodiment of the invention herein disclosed and itsnovel features will be pointed out in connection with the appendedclaim.

In the drawings:

Fig. 1 is a perspective view of an instrument embodying our inventiontogether with accessories which are normally required in thedetermination of metallic plate thickness. In the gure the instrument isshown as it may be adjusted for use in measuring the thickness of themetallic plating on one end of a kitchen sink water supply fitting.

Fig. 2 is a vertical sectional view of the instrument embodying ourvinvention.

Fig. 3 is a magnified vertical sectional view of the electrolytic cellor electrolyte holding cup.

Fig. 4 is a diagrammatical sketch of the'electrical circuit employed bythe instrument.

Like reference numerals refer to similar parts throughout the variousviews of the drawings.

The embodiment of the present invention herein shown and describedrepresents only the preferred construction of the apparatus. -It is tobe recognized that numerous changes and modications may be made withoutdeparting from the spirit and scope of the invention. Also, for purposesof illustration only, the method of measuring the thickness of chromiumplating on a plumbing fitting will be described in detail. It is to beunderstood that the method, altered by the proper choice of anelectrolyte solution, may be employed for the determination of thethickness of other metallic platings such as, for example, nickel andthe like.

Referring to Fig. 1, the apparatus comprises a suitable base I that maybe employed as a container for the necessary electrical equipment whichwill be hereinafter described in greater detail. A post 2 extendsupwardly from the base and provides a slide for the vertical adjustmentof the arm 3 which may be rigidly positioned at any desired location onthe post 2 by means of the setscrew 4. The outermost end of the arm 3 isprovided with a hub and setscrew 5 and carries the supporting member 6for the electrolytic cell 1. The cup for the cell 1 is secured to themember 6 by the setscrew 8. An electrical wire 9 and spring clip I Iprovide the anode connection between the base I and the plumbing fittingI2, the latter being the article having the plating to be measured.

Accessories shown which complete the required equipment for conducting atest include a rubber bulb and glass tube syringe I3, a stop watch I4,an auxiliary electrolyte cup I5 and a container i6 with a quantity ofsuitable electrolyte solution.

Attention is now directed to Fig. 2 in which the internal constructionof the base I and the post 2 are shown. The electrical equipmentrequired for operation `of the apparatus is conventional and comprises avoltmeter 17 permanently connected across the electrolytic cell. It hasbeen found that a Weston model 301 5-volt meter with resistance of 1000ohms per volt is suitable. A Tripplett model 225 milliammeter I8 whichhas been equipped with an extra shunt and read-z justed to furnish twoscales is also provided. The regular scale is 50 milliamperes and theextra scale has a factor of one-fourth. The electrical current issupplied by a battery of drycells I9 and the current may be suitablyadjusted by the rheostat 2I. The switch 22 is provided to open and toclose the electrical circuit, and the switch 23 is provided to cut in orto cut out, as desired, the shunt 33 which is connected to the ammeter.The electrical connections within the base I are not shown inasmuch asthe complete circuit may be traced more readily in the wiring diagramFig. 4.

The mounting post 2 is preferably tubular and contains therewithin therod 24 having a turning knob 25 secured to the uppermost end of the rod,the knob being guided by and enclosing the upper end of the post 2. Thelowermost end of the rod 24 extends through and is in screwthreadedengagement with the bottom plate 2B of the base I. This constructionmakes possible slighr, raising and lowering of one end of the base withthe consequent raising andA lowering of the electrolytic cell 1 for thepurpose to be subsequently described herein in greater detail.

Fig. 3 shows in detail the construction of the electrolytic cell 1 whichcomprises the holder or supporting member 6, a frusto-conical cup 21containing a quantity of suitable electrolyte 28, a perforated rubbersheet 29, or the like, the metallic plate 3l and the base 32.

The electrical diagram represented in Fig. 4 comprises the battery I9,the positive terminal of which is connected through the ammeter I8 tothe plating 3|. The negative terminal of the battery is connectedthrough the switch 22 and the rheostat 2i to the electrolytic cell 1.The voltmeter I1 is connected across the line. The

switch 23 and the shunt 33 are, as previously e'xplained, provided tofurnish the ammeter with an additional scale.

The principle of the instrument, embodying our invention is essentiallythat of stripping metallic plate electrolytically at a xed rate from asmall area and determining the thickness thereof from the time requiredto remove the plate. In order to fix the rate of solution of the plate,it is necessary to maintain a constant and uniformly distributed currentdensity over the area. being stripped. The instrument must be capable ofreproducing a given current density for each successive measurement ofthickness made on each kind of plating. This is accomplished in twoways, as follows: (1) For substantially flat surfaces a means isprovided for reproducing both the area and the total current; (2) Forthe measurement of plate upon a curved or irregular surface, the areaand the total current are allowed to vary somewhat, but the currentdensity is reproduced with reasonable accuracy by adjusting the voltageacross the electrolytlc cell.

For determining the thickness of chromium upon a substantially fiatsurface the following procedure has been found to be satisfactory: rlherubber sheet 29, having a thickness of about it; inch and having anaperture 34 about 1% inch in diameter therein, is placed over thesurface area upon which it is desired to measure the thickness of l'theplate. The cup 21, which is preferably made of brass and which has anopening of approximately @2f inch in diameter at its lower end, isplaced upon the rubber sheet with the said opening concentric with theaperture 34. it is essential that pressure be applied to the rubber 29by the cup 21 in order to prevent leakage of the electrolyte solution.The weight or" the apparatus may be utilized to apply the pressure inthe following manner: The rod 24 may be screwed downwardly until itsupports one end of the apparatus. The arm 3 and the holding member 6may then be adjusted to hold the cup 21 lightly upon the rubber sheet29. The rod 2li may then be screwed upwardly whereby the weight of oneend of the apparatus is placed upon the cup and rubber sheet.

An electrolyte solution which is suitable for determining the thicknessof chromium is:

Na3PO4 1.0 N Na2SO4 1.0 N

With the instrument in the adjusted position I4 started simultaneouslytherewith. The cur rent must then be immediately adjusted to a suitablevalue. It has been found that if a current of about 35.5 milliamperes isused, the thickness of the chromium in millionths of an inch isnumerically equal to the time in seconds for the electrolytic cell topenetrate the layer of chromium. When the chromium subjacent theaperture 34 has been penetrated, there is a marked change in the currentand voltage as readily observed on the ammeter I8 and voltmeter I1. Thestop watch should be stopped at this instant and the elapsed timeobserved for computation of the chromium thickness.

For curved or irregular plated surfaces a similar rubber sheet having anaperture 34 about ya; inch in diameter and a cup 21 having an opening inits lower end of about 13g inch in diameter are preferably used. Aquantity of electrolyte (about one-fourth the cup capacity) should beplaced in the cup after the apparatus has been adjusted, as previouslydescribed in connection with the procedure to be used on a fiat surface,for a test of the plate'thickness. The switch 23 should be turned to cutout the shunt 33 whereupon the ammeter I8 will read upon the smallerscale.A

The method for measuring the thickness of the chromium upon the curvedor irregular surface is identical to the method described above formeasuring the thickness of the chromium on a substantially flat surfacewith the exception that instead of adjusting the current to apredetermined value, the voltage is adjusted to a 'predetermined value.The thickness of the chromium is proportional, within reasonable limits,to the time of stripping, even though the area and current may varysomewhat with successive readings due to deformation of the rubber sheetupon the curved surfaces.

In the event that two or more layers of different metallic platings havebeen deposited one upon another and it is desired to measure thethickness of each, successive measurements may readily be made in thefollowing manner: As previously described, the thickness of the outerplate may be determined. At the conclusion of that test the switch 22should-be turned olf, the electrolytev 28 should be removed (preferablyby the syringe I3 Without disturbing the apparatus), the cell 1thoroughly flushed with distilled water, then emptied and a secondelectrolyte placed in the cup 21. The second electrolyte should be of acomposition suitable for forming an electrolytic cell with the metal ofthe second plating layer. The method of measuring the thickness of theouterplating may then be repeated for the determination of the thicknessof the second layer. Obviously, if there are stillother subjacent layersof plating, the method may be vsuccessively utilized in measuring thethickness of each layer.

'Ihe determination of metallic plating thickness which has been madeavailable by this invention is rapid, economical and convenient. Thearticle upon which measurements have been made needs only to be replatedto restore it to its original condition, thus the usual waste caused bydestruction of the article is avoided. That the results obtained areaccurate within reasonable tolerances has been proved by actualcomparative tests using other well known methods cal current and inwhich the plating serves as the anode, comprising in combination afrusto-conical container for an electrolyte, the frusto-conical form ofthe said container extending immediately upwardly and outwardly from itsopen bottom, the said container serving as the cathodeha gasket havingan aperture therethrough of a diameter equal to the diameter of a localanodic area to be measured, the said gasket aperture being ofsubstantially less diameter than the lower end of the open bottom of thesaid frusta-conical container, whereby when the said frusto-conicalcontainer is placed over the gasket aperture and the current is passedtherethrough the distribution of current over the anodic area issubstantially uniform throughout. STANLEY ANDERSON. ROBERT W. MANUEL.

